Standard Magnetic Measurements Research Articles

Origin of the fast magnetization relaxation at low temperatures in HTS with strong pinning

The temperature T variation of the normalized magnetization relaxation rate S in high-temperature superconductors (HTS) with strong vortex pinning exhibits a maximum in the low-T range. This was reported for various HTS, and the origin of the faster relaxation at low T appearing in standard magnetization relaxation measurements was usually related to specific pinning properties of the investigated specimens. Since the observed behaviour seems to be characteristic to all HTS with enhanced pinning (generated by random and/or correlated disorder), we show that the S(T) maximum can be explained in terms of classic collective vortex creep. The influence of thermo-magnetic instabilities in the low-T range is also evidenced. The collective (elastic) creep regime is generated by the T dependent macroscopic currents induced in the sample during standard magnetization measurements.

Nanopowders with superparamagnetic Fe3C particles and their annealing behaviour

AbstractTwo samples of nanopowders containing superparamagnetic Fe3C and Fe3O4/γ–Fe2O3 particles and carbon black were synthesized by the method of laser‐induced homogeneous pyrolysis of gaseous precursors. Both were characterized by XRD, HR‐TEM, Mössbauer spectrometry and standard magnetic measurements. The mean crystallite size of Fe3C was 3 nm for the first sample and 10 nm for the second sample. Fe3C phase in both samples exhibited reduced Curie temperature and smeared ferromagnetic–paramagnetic transition. After annealing of the samples at 800 °C for 30 min the Curie temperature of the recrystallized Fe3C phase in both samples was ∼214 °C (standard bulk value). Copyright © 2010 John Wiley & Sons, Ltd.

Powders with superparamagnetic Fe3C particles studied with Mössbauer spectrometry

Two nanopowders with superparamagnetic Fe3C particles were synthesised by the method of laser-induced pyrolysis of gaseous precursors. Both were characterised by X-ray diffraction, Mössbauer spectrometry and standard magnetic measurements. The mean crystallite size of Fe3C was 3 nm for the first sample and 10 nm for the second sample (Scherrer formula), i.e. it was lower than in our previously studied ferromagnetic Fe3C-based sample. Fe3C phase in both present samples exhibited by ~20 K reduced Curie temperature which is interpreted as a nanosize effect. After annealing of the samples at 1073 K for 30 minutes the Curie temperature of the Fe3C phase in both samples matched its standard bulk value. Beside Fe3C phase also Fe3O4 and carbon black were present in the synthesised samples.

Magnetic properties and environmental changes recorded in Lake Lehmilampi (Finland) during the Holocene

Standard mineral magnetic measurements, including magnetic susceptibility, ARM and IRM, were carried out on two Holocene sediment cores from Lake Lehmilampi (Finland), which span the last ~9,700 years. Paleoenvironmental changes were inferred from stratigraphic variations in the sediment mineral magnetic properties. Total organic carbon (TOC) measurements supplement the bulk mineral magnetic data. Development of Lake Lehmilampi is closely connected with Lake Pielinen, from which Lake Lehmilampi was gradually isolated ~5,066 Cal. years BP. Since the isolation of Lake Lehmilampi, its sediment has been annually laminated, with clastic-organic varves in the deep basin. Magnetic mineralogy is dominated by ferrimagnetic minerals, interpreted as magnetite throughout the sediment column, with a variable, but minor contribution of antiferromagnetic minerals. Sediment magnetic properties indicate a series of changes in sediment composition during the Holocene, which correlate well with sediment lithology and TOC. Magnetic properties differ in pre- and post-isolation sediments with respect to concentration of fine-grained magnetite, as expressed by χARM and χARM/SIRM, showing variation in response to organic matter content in post-isolation varved sediments. This may reflect authigenic bacterial production of magnetite in the weakly anoxic environment. Preliminary data suggest that magnetosome production in Lake Lehmilampi is controlled by lake productivity, and ultimately by climate.

Assessment of catalyst particle removal in multi-wall carbon nanotubes by highly sensitive magnetic measurements

The purity of the multi-wall carbon nanotubes (MWCNTs) from catalyst nanoparticles (cobalt and iron) after conventional nitric acid refluxing has been assessed by standard methods and magnetic measurements. X-ray diffraction and energy dispersive X-ray spectroscopy analyses are not suitable since the catalyst amounts quickly become beyond the detection limit of such techniques. Inductively coupled plasma spectroscopy is useful for this purpose, however it requires a large amount of sample and a long sample processing time since the MWCNTs must be completely destroyed. By contrast, magnetic measurements are a fast and nondestructive method to monitor the catalyst content, showing the existence of a small but detectable signal even for the purest sample, still far from its detection limit. Hence, magnetic measurements are extremely sensitive to detect catalyst impurities and can be used as a quick, first order, tool to evaluate the level of magnetic impurities.

Pore network geometry in low permeability argillites from magnetic fabric data and oriented mercury injections

The relationship between mineral fabric and pore network geometry anisotropy is studied for representative clay‐rich lithologies from the Callovo‐Oxfordian formation of the Andra Laboratory (Paris Basin, France), sampled from the oblique borehole‐core #EST211. Comparisons between standard magnetic fabric measurements (AMS) that give the mineral fabric, high pressure mercury injections parallel to the AMS axes, that reflect the pore structure parameter and AMS measurement after impregnation with a ferrofluid at different pressures, giving the shape of the pore networks, allow us to propose three pore models. (1) In the silt‐enriched specimens the transfer pathway is found to be controlled by vertical pathways. (2) In the carbonate‐enriched specimens the best transfer pathway is parallel to bedding, and in this plane is almost isotropic in spite of an anisotropic pore structure. (3) In the clay‐enriched specimens, at the site of the Andra Laboratory, the transfer pathway is also parallel to bedding and ∼20% higher parallel to both the magnetic lineation and the pore shape preferred orientation.

Origin of experimental order-disorder transition line curvature in theLa2−xSrxCuO4single crystal vortex system at low temperatures

We investigated the dc magnetization curves of overdoped ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}$ single crystals in the low-temperature $T$ range, with the external magnetic field $H$ oriented along the $c$ axis. It was found that the onset field for the second magnetization peak, associated with the order-disorder transition in the vortex system, increases significantly with decreasing $T$ in the low-$T$ region, where the superconductor parameters are independent of $T$. The upward curvature of the order-disorder transition line determined in standard magnetization measurements at low $T$ is explained by considering the reduction of the actual pinning energy due to the macroscopic currents induced in the sample. A simple energy balance equation for the dynamic conditions generated in the widely performed magnetization studies is proposed.

Assessing the ability of first‐order reversal curve (FORC) diagrams to unravel complex magnetic signals

First‐order reversal curve (FORC) diagrams for mixtures of different magnetic phases and bimodal distributions have been measured to examine the efficiency of the FORC method at unraveling complex magnetic signals. The FORC distributions for various magnetic minerals, including magnetite, maghemite, hematite, and goethite, and their linear additivity are assessed. Mixtures containing only hard magnetic minerals like hematite or goethite, which have relatively small spontaneous magnetizations (MS) and large magnetocrystalline anisotropies, can be adequately described by a linear addition of the two end‐members, because there are virtually no magnetostatic interactions between the phases. Mixtures dominated by softer minerals like magnetite and maghemite are more susceptible to interactions and exhibit nonlinear behavior. When a hard phase with low MS like hematite is mixed with a softer phase with high MS like magnetite, it can still be identified using the FORC technique, whereas it is impossible to do so using standard magnetic hysteresis measurements. When the weaker phase can be identified, then weak‐strong mixes add linearly; however, beyond a certain critical concentration the mineral with high MS swamps the magnetic signal and linearity breaks down. It is suggested that the FORC method is highly suitable for identifying small traces of hard magnetic minerals like hematite and goethite in the presence of minerals with high MS such as magnetite.

Nonmonotonic temperature dependence of the experimentally determined vortex-creep activation energy in disordered high-temperature superconductors

The temperature $T$ variation of the normalized vortex-creep activation energy $U*$ determined in standard magnetization relaxation experiments for ${\mathrm{Pb}}_{2}{\mathrm{Sr}}_{2}{\mathrm{Y}}_{0.53}{\mathrm{Ca}}_{0.47}{\mathrm{Cu}}_{3}{\mathrm{O}}_{8+\ensuremath{\delta}}$ single crystals with random point disorder exhibits a maximum, which moves to lower-$T$ values by increasing the external magnetic field oriented parallel to the $c$ axis. The nonmonotonic $U*(T)$ dependence is related to the change of the vortex pinning barriers involved in the creep process across the order-disorder transition in the vortex system (accompanied by the occurrence of the second magnetization peak), in a dynamic scenario. The decrease of $U*$ with decreasing $T$ in the low-$T$ region is caused by the shift of the current density $J$ range probed in standard magnetization measurements toward the critical current density, and the significant $U*(J)$ variation in the elastic-creep domain. The dynamic approach is confirmed by the behavior of highly disordered top-seeded melt-grown $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ crystals at low $T$, for which no second magnetization peak appears, and $U*$ does not depend on $T$.

Field Decay and Snapback Measurements Using a Fast Hall Probe Sensor

During beam injection the components of the magnetic field inside the magnets decay in time. At the re-start of the ramping, this decay is cancelled resulting in a fast change of the harmonics called "snapback". This sudden variation affects mainly the sextupole and decapole components and can induce significant changes in the machine chromaticity, thus causing particle beam loss. Standard magnetic measurements with rotating coils do not have a sufficient time resolution to properly characterize the snapback phenomenon. A system based on Hall probes has been developed from an existing prototype to measure with a moderate acquisition frequency (3-10 Hz) the decay and snapback of the sextupole and, for the first time, of the decapole fields. The present system also provides local measurements of these field harmonics along a wavelength of the superconducting cable twist pitch. In this paper, we describe the assembly features of this detector and of the measurement chain. The performances are demonstrated based on preliminary measurements performed on the LHC pre-series dipoles in operating conditions.

Synthesis and magnetostructural properties of two crystalline phases of [CuBr 2(sp)] (sp=(−)-sparteine)

Two crystalline phases, 1a, 1b, of [CuBr 2(sp)] complex (sp=(−)-sparteine) were obtained by direct synthesis. Crystal data: complex 1a, triclinic, P1, a=7.4715(6) Å, b=7.7082(7) Å, c=9.1435(6) Å, α=97.429(6)°, β=112.808(5)°, γ=110.666(6)°; complex 1b, orthorhombic, P212121, a=11.0563(7)(6) Å, b=11.9877(9) Å, c=12.8002(9) Å. The molecular locality of Cu(II) ion is tetra-coordinated in each case and show close values of angle and bonds length for 1a, 1b. The UV-Vis and IR spectra of 1a and 1b are very similar. 1H NMR spectra of 1a, 1b, are typical of Cu(II) paramagnetic complexes with broad lines in a range between 34 to −21 ppm. ESR spectra at 300 and 77 K of a polycrystalline sample for complex 1a show a broad signal with g=2.036. Two species Cu(II) are developed by power saturation study. For 1b the ESR spectrum at 300 K is a singlet with composition Lorentzian 20%/Gaussian 80%. At 77 K the ESR singlet is still narrower with Lorentzian 90%/Gaussian 10%. The ratio areas is A 77/ A 300=5.22. All this features indicate clearly a weak ferromagnetic Cu⋯Cu exchange interaction. Standard magnetization measurements at low temperatures show that cupric ion pairs exchange interact weakly, obtaining J 1 a =|2.56| cm −1 and J 1 b =|1.93| cm −1. The crystallographic data correlate quite well with 1H NMR results such as: the line shapes, the relative areas and position of the signals, with the hydrogen atoms-spheres that interact with the central copper ion.

Synthesis, crystal structure, weak antiferromagnetic behavior and electronic studies of novel [((−)-sparteine)(PhCO 2)(Cl)]Cu(II) complex

[((−)-Sparteine)(PhCO 2)(Cl)]Cu(II) 1 complex is obtained by direct synthesis using copper(0). 1 crystallizes in the monoclinic space group P2 1 with a=14.7355(12), b=8.9768(5), c=17.2810(10) A ̊ , β=111.916(5)°, and Z=4. The electronic spectrum of 1 shows a broad band with λ max ∼841 nm (ε=0.261 mM −1 cm −6) characteristic of a low symmetry and tetragonally distorted square pyramidal local Cu geometry. The far IR spectrum of 1 shows characteristic vibrations of Cu–Cl (260, 267 cm −1), Cu–N (436, 467 cm −1) and Cu–O (457 cm −1) bonds. The 1H NMR spectrum of 1 is typical of magnetic Cu(II) complexes with line broadening due to efficient nuclear relaxation from the metal center. ESR spectra of polycrystalline 1 at 77 K show an axial spectrum with linewidth of 58.6 G and at 300 K of 89.0 G, with areas in the ratio A 77/ A 300=2.79, indicative of antiferromagnetic order. The linewidth is reduced by 34% on going from 300 to 77 K. Standard magnetization measurements at low temperatures show an Curie–Weiss behavior with θ=−21.67 K, suggesting a weak exchange coupling interaction. The crystalline structure of 1 shows that the lattice is arranged so that the space between molecules is smaller than 40 Å 3, not enough to accommodate solvent molecules. However, the shortest Cu–Cu contact is 7.5912(8) Å.

Anomalous Hall effect as a probe of the chiral order in spin glasses.

The anomalous Hall effect arising from the noncoplanar spin configuration (chirality) is discussed as a probe of the chiral order in spin glasses. It is shown that the Hall coefficient yields direct information about the linear and nonlinear chiral susceptibilities of the spin sector, which has been hard to obtain experimentally from the standard magnetic measurements. Based on the chirality scenario of spin-glass transition, predictions are given on the behavior of the Hall resistivity of canonical spin glasses.

Synthesis of 1,2-bis(di- tert-butylphosphino)ethane (dtbpe) complexes of nickel: radical coupling and reduction reactions promoted by the nickel(I) dimer [(dtbpe)NiCl] 2

Tetrahydrofuran solutions of (dtbpe)NiCl 2 (dtbpe=1,2-bis(di- tert-butylphosphino)ethane) are reduced by KC 8 to afford the dimeric Ni(I) complex [(dtbpe)NiCl] 2 ( 1) in 73% yield. Reaction of 1 with [FeCp 2][PF 6] effects a one-electron oxidation to give the mixed-valent Ni(I,II) binuclear species [{(dtbpe)NiCl} 2][PF 6], [ 1][PF 6]. Complex 1 reacts with the radicals NO and TEMPO (2,2,6,6-tetramethyl-1-piperidine- N-oxyl) to give the diamagnetic Ni(0) nitrosyl (dtbpe)Ni(Cl)(NO) ( 2) and the Ni(II) complex (dtbpe)Ni(Cl)( O, N:η 2-TEMPO) ( 3). Reduction of the SS bond of diphenyldisulfide by 1 results in formation of the Ni(II) arylthiolate complex (dtbpe)NiCl(SPh) ( 4). The radical anions NaOCPh 2 and KN 2Ph 2 react cleanly with 1 to afford (dtbpe)Ni(η 2-OCPh 2) ( 5) and (dtbpe)Ni(η 2-N 2Ph 2) ( 6). Phenylacetylide CC bond coupling is effected by reaction of 1 with LiCCPh to form [(dtbpe)Ni( C, C′:η 2-CCPh)] 2 ( 7). In addition to standard spectroscopic (IR, NMR, EPR) and magnetic measurements, complexes 1, [ 1][PF 6], 3, and 5 were also characterized by single-crystal X-ray diffraction methods.

Field decay and snapback measurements using a fast Hall plate detector

In superconducting particle accelerators significant changes occur in tune and chromaticity during the injection of particles and their subsequent acceleration. This behavior is caused by the decay of magnetic field components in the superconducting accelerator magnets during injection and their so-called to the original hysteresis curve during the first few seconds after the start of the energy ramp. The two effects are closely related to a spatially periodic modulation of all field components along the magnet axis. In order to avoid a loss of particles the sextupole component in the dipoles has to be compensated with a very high accuracy. Standard magnetic measurements using rotating coils do not have the time resolution required to completely resolve the snapback. For this reason we have developed a fast system for sextupole measurements consisting of three Hall plates mounted on a ring. A new calibration procedure for the first time allows quantitative results. The detector is used to measure decay and snapback as a function of several parameters in the operation cycle. The results obtained help to better understand the interaction between current distribution and magnetization in the coil.

Magnetic properties of GdT 4Al 8 and GdT 6Al 6 compounds (T=Cr, Mn, Cu)

We have studied the magnetic properties of GdT 4Al 8 and GdT 6Al 6 compounds with T=Cr, Mn, Cu by means of standard magnetization and susceptibility measurements, magnetization measurements in high fields up to 35 T, and measurements of the specific heat. In these GdT 4Al 8 compounds, the T element does not carry a magnetic moment. In all these cases, the coupling between the Gd moments is fairly weak and leads to antiferromagnetic ordering at rather low temperatures. We have analyzed our data in terms of a simple mean-field two-sublattice model. Reliable values of the intersublattice coupling strength were derived from high-field measurements made on free powder particles. In the GdT 6Al 6 compounds, more than one crystallographic site is occupied by the T elements, leading to relatively strong changes in the magnetic ordering temperatures. A comparison is made with the magnetic properties of GdFe 4Al 8 and GdFe 6Al 6 compounds in which the Fe moments strongly determine the magnetic properties. High-field measurements are presented also for the latter two compounds.

Magnetoelastic and Optical Studies of Low Temperature Magnetic Relaxation in Rare-Earth Orthoaluminates with Ising Ions

The new class of magnetic materials with low temperature magnetic relaxation was studied. It was evidenced that the crystal DyA1O 3 with antiferromagnetically ordered high anisotropy rare-earth ions showed an exponential magnetic relaxation. For the first time low temperature magnetic relaxation was investigated by means of magnetostriction and rare-earth optical absorption spectra instead of standard magnetization measurements. The possible mesoscopic nature of tunneling process was suggested.

A fast sextupole probe for snapback measurement in the LHC dipoles

In superconducting particle accelerators a fast change of the magnetic field occurs during the first few seconds after the start of an energy ramp. Standard magnetic measurements using a coil rotating at 1 Hz do not have the time resolution required to completely resolve this phase, usually called snapback. For this reason we have developed a new and fast system dedicated to sextupole measurements. The basic component consists of three Hall plates mounted on a ring. In an ideal case this arrangement compensates the main dipole field and produces a signal proportional to the sextupole only. Mechanical tolerances and differences in the sensitivity of the Hall plates are compensated by instrumentation amplifiers and an in situ fine adjustment of the probe orientation. Using this hybrid compensation technique we have measured sextupole variations in an LHC dipole prototype during snapback at a rate of 5 Hz. In this paper we present details on the device and the results of our measurements.

Harmonic calculations and measurements of the irreversibility field using a vibrating sample magnetometer

The effect of the field inhomogeneity of the magnet on a vibrating sample magnetometer (VSM) measurement of a superconductor is calculated using Bean's model and Mallinson's principle of reciprocity. When the sample is centered in both the magnetic field and the VSM pick-up coils, the hysteretic signal obtained in a VSM measurement, associated with the critical current density ${(J}_{C}),$ is reduced to zero when the effective ac field caused by the sample movement penetrates the entire sample and not, as is commonly assumed, when the critical current density becomes zero. Under these conditions, an apparent phase transition is observed where the magnitude of the hysteresis drops to zero over a small field range. This apparent transition is solely an artifact of the measurement and cannot correctly be compared to theoretical calculations of the irreversibility field ${(B}_{\mathrm{IRR}}),$ which is the phase boundary at which ${J}_{C}$ is zero. Furthermore, the apparent reversible magnetization signal in high fields includes two contributions. In addition to the usual diamagnetic contribution from the thermodynamic reversible magnetization of the superconductor, there is a reversible paramagnetic contribution from the nonzero ${J}_{C}.$ Hence values of the Ginzburg-Landau parameter (\ensuremath{\kappa}) cannot be reliably obtained from standard reversible magnetization measurements using a VSM unless it is confirmed that ${J}_{C}$ is zero. Harmonic measurements using a VSM are reported. They confirm the results of the calculations. By applying a large field gradient, the hysteresis in the magnetization signal at the drive frequency of the VSM is found to drop to zero more than 3 T below ${B}_{\mathrm{IRR}}.$ We propose methods to improve measurements of ${B}_{\mathrm{IRR}}$ and \ensuremath{\kappa}. The implications of results presented for superconducting quantum interference device measurements are also briefly discussed.

About the possibility of defining a standard method for iron loss measurement in soft magnetic materials with inverter supply

Due to the increase of electromagnetic and electromechanical devices supplied by static converters, the classification and evaluation of the energetic behavior of soft magnetic materials, with nonsinusoidal supply, is becoming mandatory. In this paper, the possibility of defining a standard methodology for the specific iron loss measurement (iron losses per mass unity), with static supply, is analyzed. In particular, the paper underlines the main problems which can be found when standard magnetic measurements are carried out with nonsinusoidal sources. Pulsewidth modulation (PWM) inverters can be considered an industrial standard solution as a static supply source. As a consequence, the problems linked to the use of this kind of supply with magnetic material have been analyzed, taking into account the effects due to the PWM inverter characteristics (for example, modulation index, switching frequency, and modulation technique). In addition, in the paper, a basic structure of a standard test bench is proposed, also taking into consideration the required instrumentation.